1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to an electron gun for a cathode ray tube that can reduce a change in the size of a spot due to a change of a focus voltage and a change of current.
2. Discussion of the Related Art
Generally, a cathode ray tube includes an in-line electron gun that emits three electron beams, a deflection yoke that deflects the electron beams in a predetermined place of a screen, a shadow mask that selects the electron beams, and a screen that reproduces a picture image by colliding with the electron beams.
According to the Japanese Patent Laid-Open No. 60-51775, a typical electron beam spot becomes great if beam current increases. Therefore, to obtain a fine picture image, the beam current should be within the smaller range if possible. However, since a cathode ray tube that requires high current has a great change of current, a uni-bi potential lens structure having an improved pre-focus area in a bi potential main lens structure has been employed to reduce the spot size on the screen.
A related art electron gun for a cathode ray tube will be described with reference to FIG. 1.
Referring to FIG. 1, the related art electron gun includes a cathode K that emits three electron beams of R, G, and B, a first electrode 1 that controls the electron beams emitted from the cathode K, a second electrode 2 that accelerates a thermal electron emitted from the first electrode 1, third, fourth and fifth electrodes 3, 4, and 5 that focus the electron beams, and a sixth electrode 6 that acts as an anode.
The operation of the aforementioned related art electron gun will now be described.
If a heater provided inside the cathode K heats the cathode K, the electron beams are emitted. The emitted electron beams are controlled by the first electrode 1 that acts as a control electrode. Also, the emitted electron beams are accelerated by the second electrode 2 and focused by the third to sixth electrodes 3, 4, 5, and 6.
Meanwhile, if high current is generated from the electron gun, the current density of crossover does not increase by an increased value of the beam current due to the space charge repulsion. The current density is uniformly distributed without forming a Gaussian distribution, thereby degrading the crossover. If the crossover is degraded, the spot on the screen is degraded accordingly.
To prevent the crossover from being degraded, it is necessary to reduce the potential of the crossover, thereby reducing the space charge repulsion. To increase a voltage of the crossover, the third electrode 3 moves to the second electrode 2. Thus, the potential of the crossover increases while the space charge repulsion decreases.
However, in this case, an emitting angle α increases and thus the size Db of the electron beam in a main lens increases. As shown in FIGS. 7 and 8, if the size Db of the electron beam in the main lens increases, spherical aberration increases. In this case, a problem arises in that the size of the spot on the screen increases.
To solve such a problem, it is necessary to reduce the emitting angle after the crossover passes. Since the crossover moves over the second electrode 2 under the high current, it is difficult to reduce the emitting angle by means of the third to fifth electrodes 3, 4, and 5.
To reduce the emitting angle α after the crossover passes, another pre-focus lens may be provided between the pre-focus lens by the second and third electrodes 2 and 3 and the main lens.
As shown in FIG. 1, the pre-focus lens is formed in a uni-potential lens structure by dividing a focus electrode into the third, fourth, and fifth electrodes 3, 4, and 5 and applying the same voltage to the third and fifth electrodes 3 and 5. At this time, an electron beam through hole 41 of the fourth electrode 4 has the same size as that of an electron beam through hole 51 of the fifth electrode 5. The electron beam through hole 51 of the fifth electrode 5 is formed in a direction of the fourth electrode. The electron beam through hole 41 of the fourth electrode is greater than an electron beam through hole 31 of the third electrode 3.
In this case, the emitting angle of the electron beams entered into the main lens decreases. This decreases the size Db of the electron beam in the main lens. If the size Db of the electron beam in the main lens decreases, the spherical aberration decreases. As a result, the size of the spot on the screen decreases.
However, the aforementioned related art electron gun has several problems.
As described above, in the related art electron gun, the fourth electrode is formed in a plate shape in the pre-focus lens at the front of the main lens so that the emitting angle of the electron beams is adjusted. At this time, the fourth electrode is adjacent to the third electrode while the fifth electrode is adjacent to the fourth electrode. In forming the pre-focus lens, the electron beam through hole of the fourth electrode has the same size as that of the fifth electrode. In this case, design factors that can adjust the emitting angle are limited to each thickness of the third, fourth, and fifth electrodes, the distance d1 between the third electrode and the fourth electrode, the distance d2 between the fourth electrode and the fifth electrode, the size of the electron beam through hole of the third electrode, the size of the electron beam through hole of the fourth electrode, and the size of the electron beam through hole of the fifth electrode. As a result, to additionally adjust the emitting angle, supplementary electrodes are required among the second electrode, the third electrode, and the fourth electrode. This causes a complicated structure.